The invention relates to an inductive proximity sensor for embedding in a metal mounting plate and a method of designing the same.
The proximity sensors comprise an RLC oscillator with a sensor coil, the magnetic field of which generates eddy currents in a target. The power loss caused by these eddy currents represents a load which increasingly attenuates the oscillator as the distance between the sensor and the target is reduced.
The measure for the rate at which an oscillator dissipates its stored energy is the quality factor Q, which is 2π times the ratio of the energy stored to the energy dissipated per cycle. The maximal quality factor Qmax of the oscillating system of an inductive proximity sensor is measured in the sensor's non-embedded state and in absence of the target. In operation, the quality factor Q depreciates due to eddy current losses in the mounting plate and/or in a target. Normalizing the quality factor Q to the maximal quality factor Qmax, a relative quality factor Qrel is defined as follows:
                              Q          rel                =                  Q                      Q            max                                              (        1        )            
If no field-influencing bodies are present in the environment of the sensor, Q is equal to Qmax and therefore Qrel is equal to 1. What was mentioned with respect to the quality factor Q before, applies to the relative quality factor Qrel as well: it depreciates due to the fact that the energy dissipation per cycle is increased by eddy current losses in the mounting plate and/or the target. The amount of this depreciation is the measure for the attenuation of the oscillator:
                    Attenuation        =                              1            -                          Q              rel                                =                                                    Q                max                            -              Q                                      Q              max                                                          (        2        )            
The sensor circuit comprises a measuring circuit arranged to generate an output signal, which is a function of this attenuation.
A distinction is made between switching and analog inductive proximity sensors. The measuring circuit of a proximity switch is arranged to switch the output signal from one state to another as soon as the attenuation reaches a reference threshold, which is also called operating threshold in this context, whereas the measuring circuit of an analog proximity sensor is arranged to generate an output signal which, for attenuations exceeding a reference threshold, is a monotonic function of the oscillator attenuation.
The outer surface of an enclosure of the sensor comprises a sensing face to which the magnetic field of the sensor coil is directed by means of a core. The target distance is the distance from this sensing face to the target and the range of the sensor is the value to which this target distance needs to be reduced (by approaching the target to the sensing face) to produce a change of the sensor's output signal. The sensor range thus defined is called operating distance if the sensor is a proximity switch. The range specified in the data sheet of a sensor is called the rated range. The actual range of a sensor may deviate from its rated range due to manufacturing tolerances, for instance.
As a basis for the measurement and the specification of the sensor range, the international standard IEC 60947-5-2 defines the size and material of the target to be used. The standard target is a square mild steel plate with a thickness of 1 mm and a side length which is equal to either the diameter of the sensing face or three times the rated range of the sensor, whichever is greater.
The range of an inductive proximity sensor and its embeddability are conversely related. The greater the target distance, the smaller are the eddy current losses in the target, resulting in an increased impact of eddy current losses in the mounting plate on the total amount of eddy current losses which is determining for the oscillator attenuation.
To increase the range of a sensor, its reference threshold is reduced. In the following table, the rated operating distance of embeddable proximity switches as specified in the standard IEC 60947-5-2, which is attainable with a reference threshold of approximately 35 percent, is compared to the measured operating distance of correspondingly sized state of the art proximity switches characterized by a reference threshold of only 10 percent:
TABLE 1Operating distanceSensor sizeIEC Standard10% thresholdM5 —1.2 mmM8 1 mm2.2 mmM122 mm3.7 mmM185 mm6.5 mmM3010 mm 13.0 mm 
In this table, the size of the sensors is specified by the diameter of an externally threaded sleeve forming the enclosure of the sensors.
A long range proximity sensor is characterized by a reference threshold of 10 percent or less. By reducing the reference threshold below 10 percent, sensors with an even longer range can be realized. Long range sensors with a reference threshold of less than 5 percent are known in the art. Based on the values in the right column of the above table, the minimal range Smin of a long range sensor can be approximately determined using the formulaSmin=0.14+0.114*d14  (3)wherein d is the outer diameter of the sensor's enclosure sleeve in millimeters and Smin is the sensor range in millimeters.
Ordinary long range sensors are not fully embeddable in a steel mounting plate because the attenuation caused by the mounting plate in the embedded state almost coincides with or exceeds the reference threshold of the sensor, rendering in particular any switching type proximity sensor inoperable.
To mitigate the influence of the mounting plate, the use of a shield skirt surrounding the coil and the core in order to prevent the sensor field from reaching the mounting plate is suggested in the German patent application DE 3438998 A1. A metal layer designed to act as a shield against an alternating magnetic field is at least as thick as the skin depth of the layer material at the field's oscillation frequency and preferably thicker. The skin depth of a material can be calculated as
                              skin          ⁢                                          ⁢          depth                =                              ρ                          π              ·              f              ·                              μ                0                            ·                              μ                r                                                                        (        4        )            where                ρ is the resistivity of the material in [Ω·m],        f is the angular frequency of the field in [Hz],        μ0 is the permeability of free space in [N/A2] and        μr is the relative magnetic permeability of the material.        
For instance, the skin depth of copper at a frequency of 1 MHz is 66 μm. The operating frequency of sensors with a sensor coil having a core is preferably above 50 kHz in order to reach a long operating distance, but it is generally below 1 MHz and therefore a copper layer surrounding the core must be even thicker in order to act as a shield.